1. Field of the Invention
The present invention relates to a heat resistant magnesium alloy. More particularly, the present invention relates to a heat resistant magnesium alloy which is superior not only in heat resistance, but also in corrosion resistance, and castability.
2. Description of the Related Art
Magnesium (Mg) has a specific gravity of 1.74, it is the lightest metal among the industrial metallic materials, and it is as good as aluminum alloy in terms of the mechanical properties. Therefore, Mg has been observed as an industrial metallic material which can be used in aircraft, automobiles, or the like, and which can satisfy the light-weight requirements, the fuel-consumption reduction requirements, or the like.
Among the conventional magnesium alloys, an Mg-Al alloy, for instance AM60B, AM50A, AM20A alloys, etc., as per ASTM, includes 2 to 12% by weight of aluminum (Al), and a trace amount of manganese (Mn) is added thereto. In the phase diagram of the Mg-Al alloy, there is a eutectic system which contains alpha-Mg solid solution and beta-Mg.sub.17 Al.sub.12 compound in the Mg-rich side. When the Mg-Al alloy is subjected to a heat treatment, there arises age-hardening resulting from the precipitation of the Mg.sub.17 Al.sub.12 intermediate phase. Further, the Mg-Al alloy is improved in terms of the strength and the toughness by a solution treatment.
Further, there is an Mg-Al-Zn alloy, for instance an AZ91C alloy or the like as per ASTM, which includes 5 to 10% by weight of Al, and 1 to 3% by weight of zinc (Zn). In the phase diagram of the Mg-Al-Zn alloy, there is a broad alpha solid solution area in the Mg-rich side where Mg-Al-Zn compounds crystallize. The as-cast Mg-Al-Zn alloy is tough and excellent in corrosion resistance, but it is further improved in terms of the mechanical properties by age-hardening. In addition, in the Mg-Al-Zn alloy, the Mg-Al-Zn compounds are precipitated like pearlite in the boundaries by quenching and tempering.
In an as-cast Mg-Zn alloy, a maximum strength and elongation can be obtained when Zn is added to Mg in an amount of 2% by weight. In order to improve the castability and obtain failure-free castings, Zn is added more to Mg. However, an Mg-6% Zn alloy exhibits a tensile strength as low as 17 kgf/mm.sup.2 when it is as-cast. Although its tensile strength can be improved by the T6 treatment (i.e., an artificial hardening after a solution treatment), it is still inferior to that of the Mg-Al alloy. As the Mg-Zn alloy, a ZCM630A (e.g., Mg-6% Zn-3% Cu-0.2% Mn) has been available.
Furthermore, a magnesium alloy has been investigated which is superior in heat resistance and accordingly which is suitable for high temperature applications. As a result, a magnesium alloy with rare earth element (hereinafter abbreviated to "R.E.") added has been developed. This magnesium alloy has mechanical properties somewhat inferior to those of aluminum alloy at an ordinary temperature, but it exhibits mechanical properties as good as those of the aluminum alloy at a high temperature of from 250.degree. to 300.degree. C. For example, the following magnesium alloys which include R.E. have been put into practical application: an EK30A alloy which is free from Zn (e.g., Mg-2.5 to 4% R.E.-0.2% Zr), and a ZE41A alloy which includes Zn (e.g., Mg-1% R.E.-2% Zn-0.6% Zr). In addition, the following heat resistance magnesium alloys including rare earth element are available: a QE22A alloy which includes silver (Ag) (e.g., Mg-2% Ag-2% Nd-0.6% Zr), and a WE54A alloy which includes yttrium (Y) (e.g., Mg-5% Y-4% Nd-0.6% Zr).
The Mg-R.E.-Zr alloy and the Mg-R.E.-Zn-Zr alloy are used as a heat resistance magnesium alloy in a temperature range up to 250.degree. C. For instance, in a ZE41A alloy (e.g., Mg-4% Zn-1% R.E.-0.6% Zr), since Mg.sub.20 Zn.sub.5 R.E..sub.2 crystals are present in the crystal grain boundaries, it is possible to obtain mechanical properties which are as good as those of the aluminum alloy at a high temperature of from 250.degree. to 300.degree. C. FIG. 14 illustrates tensile creep curves which were exhibited by an AZ91C alloy (e.g., Mg-9% Al-1% Zn) and the ZE41A alloy at a testing temperature of 423K and under a stress of 63 MPa. It is readily understood from FIG. 14 that the ZE41A alloy was far superior to the AZ91C alloy in terms of the creep resistance.
However, a magnesium alloy has been longed for which has a high creep limit at further elevated temperatures and which has a great fatigue strength as well. Accordingly, an Mg-thorium (Th) alloy has been developed. This Mg-Th alloy has superb creep properties at elevated temperatures, and it endures high temperature applications as high as approximately 350.degree. C. For example, an Mg-Th-Zr alloy and an Mg-Th-Zn-Zr alloy are used in both casting and forging, and both of them have superb creep strengths when they are as cast or when they are subjected to the T6 treatment after extrusion.
Among the above-described magnesium alloys, the Mg-Al or Mg-Al-Zn alloy is less expensive in the costs, it can be die-cast, and it is being employed gradually in members which are used at a low temperature of 60.degree. C. at the highest. However, since the Mg-Al alloy has a low melting point and since it is unstable at elevated temperatures, its high temperature strength deteriorates and its creep resistance degrades considerably at high temperatures.
For instance, the tensile strength of the AZ91C alloy (i.e., one of the Mg-Al-Zn alloys) was measured in a temperature range of from room temperature to 250.degree. C., and the results are illustrated in FIG. 1. The tensile strength of the AZ91C alloy deteriorated as the temperature was raised. Namely, the tensile strength dropped below 25 kgf/mm.sup.2 at 100.degree. C., and it deteriorated as low as 10 kgf/mm.sup.2 at 250.degree. C. In addition, the creep deformation amount of the AZ91C alloy was also measured under a load of 6.5 kgf/mm.sup.2 in an oven whose temperature was raised to 150.degree. C., and the results are illustrated in FIG. 2. As can be seen from FIG. 2, the creep deformation amount of the AZ91C alloy which was as-cast reached 1.0% at 100 hours and the creep deformation amount of the AZ91C alloy which was further subjected to the T6 treatment reached 0.6% at 100 hours, respectively.
Further, since the AZ91C alloy (e.g., Mg-9% Al-1% Zn) of the Mg-Al-Zn alloys has the high Al content, it gives a favorable molten metal flow and accordingly it is superior in castability. However, since alpha-solid solution crystallizes like dendrite during the solidifying process, the AZ91C alloy suffers from a problem that shrinkage cavities are likely to occur. The shrinkage cavities often become origins of fracture. FIG. 11 is a microphotograph and shows an example of a metallic structure which is fractured starting at a shrinkage cavity. FIG. 12 is a schematic illustration of the microphotograph of FIG. 11 and illustrates a position of the shrinkage cavity.
Furthermore, since the Mg.sub.17 Al.sub.12 compounds crystallize in the grain boundaries in the Mg-Al or Mg-Al-Zn alloy and since the compounds are unstable at elevated temperatures, the high temperature strength of the alloy deteriorates and the creep resistance thereof degrades considerably at high temperatures. FIG. 13 illustrates tensile creep curves which were exhibited by the AZ91C alloy (e.g., Mg-9% Al-1% Zn) at testing temperatures of 373 K, 393 K and 423 K and under a stress of 63 MPa. It is readily understood from FIG. 13 that the creep strain of the alloy increased remarkably at 423 K.
Moreover, the AZ91C alloy was subjected to a bolt loosening test, and the results are illustrated in FIG. 4. In the bolt loosening test, a cylindrical test specimen was prepared with an alloy to be tested, the test specimen was tightened with a bolt and a nut at the ends, and an elongation of the bolt was measured after holding the test specimen in an oven whose temperature was raised to 150.degree. C. under a predetermined surface pressure. Thus, an axial force resulting from the expansion of the test specimen is measured directly in the bolt loosening test, and the elongation of the bolt is a simplified criterion of the material creep. As illustrated in FIG. 4, the aluminum alloy and an EQ21A alloy including R.E. exhibited axial force retention rates of 98% and 80%, respectively, after leaving the test specimens in the 150.degree. C. oven for 100 hours under a surface pressure of 6.5 kgf/mm.sup.2. On the other hand, the AZ91C alloy of the Mg-Al-Zn alloys exhibited an axial force retention rate deteriorated to after leaving the test specimen under the same conditions.
The ZCM630A alloy (i.e., the Mg-Zn alloy) is less expensive in the costs, and it can be die-cast similarly to the AZ91C alloy (i.e., Mg-Al-Zn alloy). However, the ZCM630A alloy is less corrosion resistant, and it is inferior to the Mg-Al alloy in the ordinary temperature strength as earlier described. This unfavorable ordinary temperature strength can be easily noted from FIG. 1. Namely, as illustrated in FIG. 1, the strength of the ZCM630A alloy was equal to that of the AZ91C alloy at 150.degree. C., and it was somewhat above that of the AZ91C alloy at 250.degree. C. As illustrated in FIG. 2, although the ZCM630A alloy exhibited creep deformation amounts slightly better than the AZ91C alloy did when the test specimens were subjected to a load of 6.5 kgf/mm.sup.2 and held in the 150.degree. C. oven, it exhibited a creep deformation amount of approximately 0.4% when 100 hours passed. Thus, it is apparent that the ZCM630A alloy is inferior in terms of the heat resistance.
The EK30A or ZE41A alloy (i.e., the magnesium alloy including R.E.) and the QE22A or WE54E alloy (i.e., the heat resistance magnesium alloy including R.E. ) give mechanical properties as satisfactory as those of the aluminum alloy at elevated temperatures of from 250.degree. to 300.degree. C. However, as aforementioned, their ordinary temperature strengths are deteriorated by adding R.E. This phenomena can be seen from the fact that the ZE41A alloy exhibited a room temperature strength of about 20 kgf/mm.sup.2 as illustrated in FIG. 1.
Therefore, in the EQ21A (or QE22A) alloy and the WE54A alloy, Ag and Y are added in order to improve their room temperature strengths as well as their high temperature strengths. However, these elements added are expensive and deteriorate their castabilities.
In addition, in the magnesium alloys with R.E. added, there arise micro-shrinkages which result in failure. Hence, in the Mg-R.E. alloy, Zr is always added so as to fill the micro-shrinkages and make a complete cast mass. However, the addition of Zr results in hot tearings, and the Mg.sub.20 Zn.sub.5 R.E..sub.2 crystals deteriorate the flowability of the molten metal. Accordingly, it is not preferable to add Zr to the magnesium alloys in a grater amount, because such a Zr addition might make the magnesium alloys inappropriate for die casting.
Moreover, as above-mentioned, the Mg-Th alloy is excellent in terms of the high temperature creep properties, and it endures applications at temperatures up to approximately 350.degree. C. However, since Th is a radioactive element, it cannot be used in Japan.
As having been described so far, there have been no magnesium alloys which are excellent in the high temperature properties and the creep properties, which can be die-cast, and which are not so expensive in the costs. Specifically speaking, the AZ91C alloy of the Mg-Al-Zn alloys is superior in the castability, but it is inferior in the high temperature strength and the creep resistance. The ZE41A alloy of the magnesium alloys including R.E. is superb in the heat resistance, but it is poor in the castability.
Further, AZ91D alloy, one of the Mg-Al-Zn alloys similar to the AZ91C alloy, is good in terms of castability, corrosion resistance and tensile strength at room temperature and up to 150.degree. C., but it is inferior in terms of creep resistance at temperatures of 100.degree. C. or more. In the case that the creep resistance is low at elevated temperatures, there arises a problem in that component parts made of such alloys exhibit deteriorating tightening forces (i.e., axial forces) at the portions, for instance at the portions tightened with a bolt, when the temperature is raised during their service. When the component parts are produced by die casting, this problem is particularly notable.
The aluminum contained in the magnesium alloys forms Mg.sub.17 Al.sub.12 crystals during the solidification. When the cooling rate is as fast as die casting, there arise the areas (i.e., the dendritic cells) adjacent to the grain boundaries, areas which contain the solute atoms (e.g., aluminum atoms) prior to the crystallization in high concentrations. Due to the presence of these unstable aluminum atoms, the grain boundary diffusion is active in the environment where the temperature is elevated, and accordingly it is believed that the unstable aluminum atoms facilitate the creep deformations.